Radiation detectors used for imaging applications like spectrally resolving computed tomography (CT) make use of a direct conversion material, such as cadmium telluride (CdTe), cadmium zinc telluride (CaZnTe) or silicon (Si), that is disposed between a cathode and an anode, with a voltage applied across the cathode and the anode. X-ray photons that illuminate the cathode transfer energy to electrons in the direct conversion material, which creates electron/hole pairs with the electrons drifting towards the anode. The anode, in response, produces an electrical signal that is further processed in order to measure photon counts in one or more energy bins. This is usually done by means of an amplifier which amplifies the electrical signal, a pulse shaper which processes the amplified electrical signal and produces a pulse having a peak amplitude or height that is indicative of the energy of the detected radiation, an energy discriminator which compares the height of the pulse with one or more thresholds, a counter which counts for each threshold the number of times the leading edge of a pulse crosses the threshold, and an energy binner which bins the photon counts into energy ranges, thereby spectrally resolving the detected radiation. The energy binner can, in principle, be implemented in hardware; more typically, however, it is realized by an external software component that reads-in the photon counts from the counter.
D. S. Bale and C. Szeles, “Electron transport and charge induction in cadmium zinc telluride detectors with space charge build up under intense x-ray irradiation”, Journal of Applied Physics, Vol. 107, No. 11 (2010) describe that wide band gap semiconductor radiation detectors using a direct conversion material are susceptible to a time dependent positive or negative charge build up that increases under x-ray illumination.
Due to this trapped and time dependent charge (which can be a positive or a negative charge), the amount of x-ray generated electrons is reduced during travel to the anode. Thus, the detected pulse height for a given x-ray energy changes leading to a time dependent photon count rate for a given threshold. This time instability can be very degrading to the quality of generated images, for instance, it can lead to ring artifacts in the images with time dependent strength.
H. Ding and S. Molloi, “Image-based spectral distortion correction for photon-counting x-ray detectors”, Medical Physics, Vol. 39, No. 4 (2012) discloses an image-based spectral distortion correction for photon-counting x-ray detectors. The authors performed experiments where they calibrated a Cadmium-Zinc-Telluride (CZT) photon-counting detector with five energy thresholds using BR12 phantoms of various thicknesses. The measured counts were compared with simulation results and for each ith energy bin a calibration function was derived which translates the measured counts into simulated ones.
WO 2008/013663 A2 discloses a gain/lag artifact correction algorithm and software. A total number of traps to be filled in a detector of an imaging system is estimated based on a measured signal sensed by the detector. The measured signal is adjusted based on the estimated total number of traps and a current trap state of the detector. The trap state of the detector is subsequently updated.